The invention relates to an apparatus for keying in electron beams, appropriate for a raster electron scan microscope, an electron beam recording device, and the like. Such apparatus comprises a deflection system for the deflection of the electron beam and controllable as a function of time for deflecting the beam to effect a scanning operation. A velocity modulating effect adapted to the beam velocity is used for controlling deflection.
The use of an electron beam keying apparatus is known from the state of the art in an electron beam system of a raster electron scan microscope. The actual apparatus for keying in the electron beam is a deflection system by which the electron beam is deflected in the regular case between the cathode and the raster deflection field over an aperture lens and/or it is deflected into and out of said aperture lens. Such a deflection system preferably is mounted behind the first anode, that is behind the principal electron beam (viewed from the cathode). Thereby it is at least proximal to the area of the beam focusing.
Moreover, it is a customary practice to carry out with a raster electron scan microscope potential contrast measurements, particularly at integrated circuits of the semiconductor type. It is possible to record with these readings potential courses electron-optically, for example in conductor tracks, whereby a particular interest exists in potential changes at very high frequencies and impulse currents of high frequencies. Because the evaluation of the secondary electron emission utilized with potential contrast measurements with electron beams as such is at least inappropriate for recording very rapid processes, for example with 10.sup.7 to 10.sup.9 cycles, true in time, the so-called stroboscope principle has been applied which is known from high frequency measuring techniques under the term "sampling". Thereby a repeated correlated scanning of a desired spot takes place, whose potential contrast is to be determined. This stroboscopic scanning then is repeated at a slight phase shifting, so that, in line with the type of a sampling method the entire, very high-frequency potential course can be determined, although in the measuring direction a relative time carrier effect is involved, namely in connection with the secondary emission and the collection of the secondary electrons.
This stroboscope or sampling method requires the production of extremely short electron beam impulses in the raster electron scan microscope, whose phase then is to be shifted moreover, still in comparison with the AC signal in the sample to be examined. This phase shifting is tantamount with a shifting of the impulse moment of the electron beam.
Such short electron beam impulses are realized according to known practical operation with the use of an aperture lens and such a beam deflection apparatus which basically is provided with two elements, placed opposite each other and extending in the axial direction of an electron beam, and together forming a deflection capacitor, whereby, however, a deflector plate is opposed by an installation which is a traveling wave installation with interdigital structure. In said interdigital structure the wave of the deflecting signal with which this impulse scanning deflecting installation is to be impinged moves at the same speed as the beam electrons passing through this deflecting capacitor. An interdigital structure in the meaning of the invention relates to a finger conduit to be impinged with the signal and interdigitated with a second finger conduit which is grounded.
Additional details can be obtained from the printed publications IEEE Transactions on Electron Devices, volume ED-19, No. 2 (1972), page 204-213 and Scanning Electron Microscopy (1973) (part I), Proc. of the Sixth Annual Electron Microscope Symposium, IIT Research Institute, Chicago, Ill. 60616, USA (April 1973) and Scanning Electron Microscopy (1976) (Part IV) Proc. of the Workshop on Microelectronic Device Fabrication and Quality Control with SEM, IIT Research Institute (April 1976). Such a meander conduit with interdigital structure is shown particularly in FIG. 1 of the article appearing in the first-named printed publication IEEE Transactions on Electron Devices. These cited references also show the manner of operation of a known raster scan electron microscope and/or of the deflection system with traveling wave included in the electron beam system of such a microscope.
One disadvantage of a deflection system known from such prior art consists in that the transverse deflection field in relation to the electron beam axis prevailing between the meander structure and the opposite electrode necessarily also has a longitudinal electrical field component which thus causes an acceleration or retardation of the electron beam. This leads to a defocusing of the electron beam focused as such extremely sharply on the sample to be tested, such defocusing occuring in operation as the electric deflection signal is varied to produce deflection.